Separation measurement method and devices employing diffraction waves

ABSTRACT

A method is disclosed for measuring the separation between a surface and a member. The member includes diffraction wave producing means such as an object boundary. Incident electromagnetic waves are directed at the diffraction wave producing means before and after reflection from the surface and an interference pattern is produced. Analysis of this interference pattern gives the separation.

mite States atet' 1191 Pryor et al.

[ 1 SEPARATION MEASUREMENT METHOD AND DEVICES EMPLOYING DIFFRACTIONWAVES [76] Inventors: Timothy R. Pryor, 5423 York Ln.,

Bethesda, Md. 20014; Omer L. I-lageniers, 357 Rosedale Ave., Windsor,Ontario, Canada 122 Filed: May 30,1972 21 Appl.No.: 257,801

52 11.s.c1. 356/111, 350/162 R [51] Int. Cl. G01b 9/02 [58] Field ofSearch 356/106-113; 350/162 [56] References Cited UNITED STATES PATENTS3,518,007 6/1970 Ito. 356/159 1 Jan. 15, 1974 Primary Examiner-DavidSchonberg Assistant Examiner-Conrad Clark Att0rney-R0berts B. Larson eta1.

[57 l ABSTRACT A method is disclosed for measuring the separationbetween a surface and a member. The member includes diffraction waveproducing means such as an object boundary. Incident electromagneticwaves are directed at the diffraction wave producing means before andafter reflection from the surface and an interference pattern isproduced. Analysis of this interference pattern gives the separation.

14 Claims, 3 Drawing Figures BACKGROUND OF THE INVENTION This inventionrelates to subject matter disclosed in U.S. Pat. No. 3,664,739 and toco-pending applications Ser. No. 253,421, filed May 15, 1972 and Ser.No. 256,099, filed May 23, 1972, by Timothy R. Pryor, which are hereinincorporated by reference.

The quality and flatness of surfaces is presently checkedwhere possibleusing an interferometer Hat and suitable optical system. Using thistechnique, deviations in surfaces of small fractions of a wavelength oflight can be determined. When surface dimension exceeds approximately 10cm however, the cost of the necessary reference flat increases rapidly,and becomes prohibitive for most applications. In addition, when thesurface is not sufficiently reflective to normally incident light,interferometry cannot be used at all. In such cases, it becomesnecessary to use autocollimator or other techniques to define anaccurate straight line between two points on the surface, takingreadingsbetween a large number of sets of points. In other words, when presentinterferometric techniques become impractical, high quality surfacemeasurement becomes difficult indeed.

In stress analysis, research, and nondestructive testing, it is often ofinterest to study the minute deflections of a member surface underapplied load, be it thermal, mechanical or whatever. Holographictechniques have been much in vogue of late and can achieve the very highresolution of displacement required, though at a considerable cost inboth time and moneylln addition, holography cannot in general deal withmany of the larger displacements found in practice, its range beinglimited practically to 0.025 mm, approximately.

Another problem in practice is the determination of straightness andquality of object boundaries, such as razor blade edges, roller bearingsurfaces and the like. No suitable technique has existed heretofore forthis purpose capable of easily determining these quantities in a highlyaccurate manner.

Accordingly, it is an object of the invention to provide a highlyaccurate, easily usable method for determining the separation between anobject and a surface, such that if the position of one is known, theposition of the other may be determined.

It is a further object of the invention to provide a means for producinga visually interpretable pattern "whose intensity distribution indicatesthe separation of a plurality of positions along aline.

It is a still further object of the invention to provide means for veryaccurately sensing small displacements of a member relative to a surfacein a quasi-digital manner.

BRIEF SUMMARY OF THE INVENTION manner that incident electromagneticradiation is reflected from the surface onto the diffraction waveproducing means of the member, to form a second diffraction wave, thefirst and second diffraction waves interacting to form an interferencepattern, and determining from the interference pattern, the separationbetween the member and the surface. Suitable apparatus for the methodcomprises means for directing electromagnetic radiation onto thediffraction wave producing means of the member to form a firstdiffraction wave, means for directing electromagnetic radiation onto thesurface in such a manner that incident electromagnetic radiation isreflected from the surface onto the diffraction wave producing means ofthe member, to form a second diffraction wave, the first and seconddiffraction waves interacting to form an interference pattern, and meansfor detecting a portion of the interference pattern for determining theseparation between the member and the surface.

DETAILED DESCRIPTION There follows a detailed description of a preferredembodiment of the invention, together with accompanying drawings.However, it is to be understood that the detailed description andaccompanying drawings are provided solely for the purpose ofillustrating a preferred embodiment and that the invention is capable ofnumerous modifications and variations apparent to those skilled in theart without departing from the spirit and scope of the invention.

FIG. 1 is a diagrammatic side elevation view of an embodiment of theinvention.

FIG. 2 is a diagrammatic perspective view of an alternative embodimentof the invention utilized to produce a visually interpretable patternproportional to the deflection of a member along a line.

. FIG. 3 is a diagrammatic side elevation of a further embodiment of theinvention employing a diffraction grating and providing a quasi-digitaloutput.

While practicing the'inventions described in the copending applicationsdescribed above a method was sought by which object reference boundariescould themselves be checked. This invention is the result, and it iscapable not only of checking the quality of straight edges and the like,but also of determining the contour or deformation of surfaces. Inaddition, the concepts herein may be used with virtually all detectionsystems discussed in the above referenced patent applications, allowingsimilar construction of practical measurement and transducer deviceshaving outstanding accuracy, range and stability, together withquasi-digital outputs, if desired.

The invention functions in essence by using waves diffracted by adiffraction wave producing means to interfere with waves diffracted bythe same diffraction wave producing means after an initial reflectionfrom a surface. A typical example results when an edge or other boundaryof an object acts as the diffraction wave producing means, thediffracted waves emanating effectively from the edge when waves areincident thereon. In a recently published scientific paper,Diffractographic Dimensional Measurement, herein incorporated byreference, we have called this the Bounce-off Diffractographic method(Applied Optics, Feb; 1972). Other examples of diffraction waveproducing means are discussed below, and in the above referenced patentand applications.

In the embodiment of the invention shown in FIG. 1, an edge boundary ofmember 11 is adjacent and locally parallel to surface 12. Surface 12 issubstantially flat and is reflective to the wavelength A ofmonochromatic, spatially coherent electro-magnetic radiation from laserwavesource 13 incident on both edge and surface. For angles of incidenced) greater than roughly the Fraunhofer type interference pattern 14produced at a distance R much greater than the surfaceedge separation w,is effectively identical in the region near the axis of reflectedradiation to that produced by a slit of width 2w illuminated at anangle 1) to its normal. The term interference pattern has been chosenrather than diffraction pattern to coincide with the terminology of theabove-identified applications and to more definitively describe thephenomena involved.

The criterion R w need not be invoked if a lens is used to form thepattern. The intensity distribution of the pattern is, in view of thesimilarity of the present pattern to that produced by a slit,proportional to sinB/B where,

and the resulting fringe minima lie whereever B in, t 2 1r, 13w etc. For(b fixed, the separation w may be determined by measuring the angle 7'from a line parallel to the surface in plane of the diagram at which oneor more of the fringes lies.

When the fringes lie at small angles 0 away from the axis 15 located aty 41), a trigonometric approximation may be made allowing the use of thefollowing approximate relation for the condition when fringe minima arelocated at angles 0;

where w equals 2w, cos (1). Relation (2) is the same as relations (3) ofthe above-identified patent applications, and for this reason all meansfor interpreting such interference patterns to give w or changes in wdiscussed therein may be used. As mentioned in the referencedapplications, analysis of the interference pattern allows w to beresolved to under 0.2 microns using the naked eye, and to less than 0.01microns using photoelectric detection. Furthermore, the means by whichthe interference pattern is generated and readout may be extremelystable and quasi-digital readout means may be provided which provide anumber of counts proportional to the change in w, and thence w,..

As is apparent, changes in w, are twice magnified as changes in w, andthus this bounce-off version is twice as sensitive as the transmissiontype described in the referenced applications. This double sensitivitycan be of importance in construction of high resolution displacementtransducers.

As shown in FIG. 1, the invention constitutes the most accuratepractical means of checking straight profiles known to us, since thesurface acting as the reference for the tested edge or other objectboundary can be an interferometer flat of flatness deviation less thanl/lOO A. In this case, all changes in the interference pattern asdifferent y locations along the edge are illuminated may be considereddue to the object edge, and the resulting changes in w, are determinableby analysis of the pattern with great accuracy (in fact, near theaccuracy of the flat itself). The technique is especially useful whendone along a whole length of boundary as in the case shown in FIG. 2.

Where the object is -a cylindrical roller bearing or other member whoseboundary has much less curvature than the wedged edge boundary 10 shownin FIG. 1, smaller angles (b are required if the waves are all to bediffracted from essentially the same position on the boundary. Thiscriterion is not necessary for the function of the invention, butsimplifies analysis of the pattern and the information derivedtherefrom.

When angles 4) less than 15 (approximately) are used in apparatusarranged as in FIG. 1, we found that a new type of fringe pattern beginsto be noticeable (though the effect is present for all angles (b). Wecall them crossed fringes" and their origin is phenomenologicallyexplained in our above referenced paper on Diffractographic DimensionalMeasurement". They are manifested as fringes spaced in the y directionand perpendicular to the surface. At any given' y location, a crossedfringe minima occurs when the value of w, at the location is such that2w, sinrb n A Such a crossed fringe case is illustrated and furtherdescribed in relation to FIG. 2.

' As mentioned previously, other types of diffraction wave producingmeans may also be used. Consider for example the replacement of member11 in FIG. 1 with small cylindrical object 16, shown in dotted lines. Inthis case, two object boundaries are presented to the incoming andreflected wavefronts and the interference pattern 14 becomes modified byan envelope proportional to the individual object width, and has fringemaxima of four times the intensity of the pattern resulting from use ofmember 11.

While of less general utility than the single boundary case, such anarrangement may be used to check the straightness of wires, strips andother such members with two parallel or nearly parallel boundaries. Inaddition, a flat surface may be checked against a tensioned wirereference in a manner analagous to FIG. 2 below.

Shown in FIG. 2 is an embodiment for determining the deflection of allpoints along a line 30 of a corner loaded cantilever plate 31 utilizingthe invention to determine separation between a fixed reference straightedge 32 and the surface of the plate. In a manner analagous to thatdescribed in the above referenced applications, an incident He-Ne gaslaser beam 33, of wavelength 6328A, is fanned by cylinder lens 34 alongthe whole length of plate desired, producing a Fraunhofer type profileinterference pattern 35 on screen 36.

Since the plate surface is in this example surface ground steel, it isnecessary to use a grazing incidence angle 4) of a few degrees to obtaina satisfactory reflection at the visible wavelength used. Thus thecrossed fringes 37 mentioned above are very much in evidence, withspacing between any two such fringes proportional to the local rate ofchange in w,, that is to the slope of the deflected plate. The crossedfringes can themselves therefore be used to measure profile changes, andif easily visible crossed fringes characteristic of small angles 4a arepresent, this represents a generally more accurate way than using theordinary interference fringes,

which are themselves modified by the crossed fringes. If one does chooseto use the ordinary fringes in this case, it is much preferred that onlythe central maximum region be used, as this is least affected by thecrossed fringe phenomena.

The crossed fringe information can further be interpreted in a mannersimilar tophotoelastic or holographic fringes by locating a nodalposition which undergoes no deflection (such as the position 0 where thecantilever plate of FIG. 2 joins the wall 38) and counting the number qof crossed fringes between the y position in question and the nodalposition. Using equation (3) therefore, the deflection of the observedlocation on the plate relative to the nodal or zero location (or anyother chosen location) is a, w, 11);, qh/Zsimb where q equals thedifference in crossed fringe order numbers n. This allows rapid andaccurate determination of profiles without measuring fringe spacings orangular locations.

The same equation (4) results if WT is considered to be some initialspacing between a diffraction wave producing means and a final w,position, perhaps indicative of displacement of an elastic memberdue toa variable such as force, pressure etc. Since equation (4) is of thesame form as those equations for count sensitivity N of a quasi-digitalphotoelectric detection system given in the above-referenced patentapplications, a similar detection scheme may be used with the crossedfringes.

Consider photodetector 39 connected to amplifier 40 and feeding-counter41 in FIG. 2. The photodetector is arranged to detect a narrow range ofy values, but extends over a sufficient distance in the x direction toavoid undue fluctuations in detected intensity as the ordinary fringesmove on and off the detector face in the x direction, with changes inw,. From equation (4), this detector will produce one full crossedfringe count every time w, changes by h/2sin, and a quasi-digitaldisplacement sensor results. Similar to methods discussed in theabove-referenced applications, two detectors may be used to produceoutput signals in phase quadrature if each is located in a patternproduced using thesame d; values but different initial separations ii,In this manner, both detectors see the same number of crossed fringesfor any change in w,, though their output signals are 90 out of phase.

The quasi-digital crossed fringe count producing detection systemdescribed above has two advantages over those of the above-referencedapplications. First, fringe spacing is the same for all values w,- andcan be quite large. This means that a detector of considerable width inthe y direction can be used if, unlike FIG. 2, the boundary and surfaceare virtually parallel, as would be the case in self-contained uniaxialdisplacement transducers. In addition to the increased intensitydetected in the y direction, the detector also extends over aconsiderable distance in the x direction as well, and thus a largepercentage of all radiation in the total interference pattern isutilized by the detection system. Fringe contrast however, is not ashigh and decreases as w increases, essentially negating this advantage.

The characteristics just described are similar in some respects to thoseexhibited by a classical fringe counting interferometer. Thus, thecrossed fringed version of the invention in this fringe counting formcan be considered an interferometer of sorts, possessing highermechanical stability and of vdetuned sensitivity. Another embodiment ofthe invention is shown in FIG. 3.

There are several additional considerations of interest. First, thesurface has been assumed nearly flat in the region of the boundary inorder to obtain the above equations. Fresnel type diffraction effectsare observed when appreciable surface curvature is present in the zdirection and this complicates analysis. The surface has been furtherassumed highly reflecting, and this is satisfied for some surfaces onlyat grazing angles, i.e. very small values of d). When such angles areused, crossed fringes invariably result and the zone of reflection fromthe surface in the z direction becomes much longer than separation w,.To obtain simple analysis therefore, all points in this zone must beeffectively on a straight line in the z direction that is, the surfacemust be locally flat in the z direction over the reflecting region.

The uses of bounce-off diffractography are several. In the FIG. 2example, a very simple and highly accurate means of checking surfaceflatness results. This is particularly useful where large surfaces areencountered, such as surface tables, machine ways and the like. In thesecases, the commonly used interferometer flat technique cannot be usedand it is considered that the disclosed invention constitutes the mostaccurate technique available. Obviously, it is essential that thestraight reference edge positions be known, and for very large surfacesthis implies considerable sophistication in the construction of suchedge members, or in some cases the use of a tensioned wire as mentionedabove.

In a similar manner, surfaces which have an arbitrary though nearly flatcurvature can be contoured as well, using a straight edge reference.Since measurement can be accomplished at very high speeds, surfacecontours of moving material can be done on-the-fly as well, as discussedin the referenced patent applications. Where curvature becomes toogreat, a point by point analysis may often be made by using a contouredreference edge and scanning an incident laser beam down the nearlyparallel aperture so formed, noting that the surface now need only benearly flat in the local region illuminated.

Where the diffraction wave producing means and the surface are divergentmore than a few degrees from parallel over a local region, it isnecessary to employ the cylinder lens technique described in theabovereferenced applications whereby a cylinder lens causes diffractedwaves to rapidly diverge in the y direction from a very small focal lineregion on the diffraction wave producing means. Such techniques may alsobe used to produce microprofiles of smallregions as well.

Another consideration is that a liquid may form the surface. Thus, smallundulations in the surface of liquids may be studied using theinvention, or liquids may serve as references either because they areflat in a non-excited state or because their plane is alwaysperpendicular to the earths gravitational field.

As mentioned above, when ordinary non-polished surfaces are used (steel,cement, etc) grazing incidence is required to obtain suitablereflectance, at least when visible electromagnetic waves are used. Whilemost useful, such visible waves are not at all required by the techniqueand when infra-red waves are used, much better reflection results fromsuch surfaces, thereby producing better quality diffraction patternsand/or negating the grazing incidence requirement.

For checking boundary profiles versus a flat mirror an advantage of theinvention relative to that disclosed in the above-referencedapplications, is that no line up is required of two edges (the testboundary and a reference boundary) in the same plane, usually itselfperpendicular to thelaser beam propagation direction.

Illustrated in FIG. 3 is an embodiment of the invention employing adiffraction grating diffraction wave producing means 50 positioned adistance w, from flat mirror surface 51, and illuminated by plane waves52. By choosing the period of the grating to be such that a gratingorder is produced in the direction of the grating normal 53, a simplefringe counting type displacement transducer of extreme sensitivityresults.

Considering the operation of the device in more detail, waves directlyincident at an angle on the grating are diffracted in a fan of gratingorders, spaced about the grating normal 53, whose position angle y fromsaid normal is given by the equation d (sin4 siny) m )t where d is thegrating period, and m is the grating order number. Similarly, incidentwaves reflected from the surface strike the grating at the sameincidence angle 5, measured in the opposite sense relative to the grating normal. However, since 'y must also be measured in the oppositesense when applying equation (5) to this case, the grating ordersproduced by both direct and This equation indicates that for values ofsin less than one half, the device is more sensitive than a conventionalfringe counting interferometer. Range is, however, more limited due tothe large incident beam width and mirror length required for largevalues of w,, when (11 is small. Note that the'device is insensitive tograting movements in a plane parallel to the surface.

The grating period and (12 may be chosen such that the grating order mdesired lies at 7 =0, parallel to the surface. This is a preferredembodiment, since unlike any other arrangement, this does not requirethat one of the two interfering grating orders be itself refleted fromthe surface. Some reflection of course may occur at small w, values,since the grating order has a small angular spread, but this is notrequired since the same angular spread of the orders allows theunreflected portions to interfere. Crossed fringes may also be formed inthis case. A detector 54 is shown in FIG. 3 located at this preferredposition.

Where grazing angles (1: are used, extremely high sensitivity resultsfrom a small simple and rugged package.

What is claimed is:

l. A method for determining the separation of a surface and a memberincluding diffraction wave producing means comprising the steps of:

directing electromagnetic radiation onto said diffraction wave producingmeans of said member to form a first diffraction wave;

directing electromagnetic radiation onto said surface in such a mannerthat incident electromagnetic radiation is reflected from said surfaceonto said diffraction wave producing means of said member, to form asecond diffraction wave, said first and second diffraction wavesinteracting to form an interference pattern; and

determining from said interference pattern, the separation between saidmember and said surface.

2. A method according to claim 1 wherein electromagnetic radiation issubsequently directed onto said diffraction wave producing means andonto said surface to form a further interference pattern at a later timeand wherein at least a portion of said further interference pattern iscompared with at least a portion of the earlier produced interferencepattern to determine changes in the separation of said member and saidsurface.

3. A method according to claim 2 including the step of detecting aportion of an interference pattern at a spatial position fixed withrespect to said diffraction wave producing means or said surface wherebyany change in an interference pattern caused by changes in separation ofsaid member and said surface is indicated by the detection ofinterference pattern fringes moving relative to said spacially fixeddetecting position.

4. A method according to claim 1 wherein said diffraction wave producingmeans comprises an object boundary.

5. A method according to claim 1 wherein said electromagnetic radiationis simultaneously directed onto a plurality of points along saidsurface.

6. Apparatus for determining the separtion of a surface of a memberincluding diffraction wave producing means comprising:

means for directing electromagnetic radiation onto said diffraction waveproducing means of said member to form a first diffractionwave;

means for directing electromagnetic radiation onto said surface in sucha manner that incident electromagnetic radiation is reflected from saidsurface onto said diffraction wave producing means of said member, toform a second diffraction wave, said first and second diffraction wavesinteracting to form an interference pattern; and

means for detecting a portion of said interference pattern fordetermining the separation between said member and said surface.

7. Apparatus according to claim 6 including detection means located at aspatial position fixed with respect to said diffraction wave producingmeans or said surface whereby any change in an interference patterncaused by changes in separation of said member and said surface isindicated by the detection of interference pattern fringes movingrelative to said spacially fixed detection means.

8. Apparatus according to claim 6 wherein said diffraction waveproducing means comprises an object boundary.

9. Apparatus according to claim 6 wherein said member including saiddiffraction wave producing means is 12. Apparatus according to claim 6wherein said diffraction wave producing means comprises an object havingtwo boundaries.

13. Apparatus according to claim 6 wherein said diffraction waveproducing means comprises a diffraction grating.

14. A method according to claim 1 wherein said surface is moving.

1. A method for determining the separation of a surface and a memberincluding diffraction wave producing means comprising the steps of:directing electromagnetic radiation onto said diffraction wave producingmeans of said member to form a first diffraction wave; directingelectromagnetic radiation onto said surface in such a manner thatincident electromagnetic radiation is reflected from said surface ontosaid diffraction wave producing means of said member, to form a seconddiffraction wave, said first and second diffraction waves interacting toform an interference pattern; and determining from said interferencepattern, the separation between said member and said surface.
 2. Amethod according to claim 1 wherein electromagnetic radiation issubsequently directed onto said diffraction wave producing means andonto said surface to form a further interference pattern at a later timeand wherein at least a portion of said further interference pattern iscompared with at least a portion of the earlier produced interferencepattern to determine changes in the separation of said member and saidsurface.
 3. A method according to claim 2 including the step ofdetecting a portion of an interference pattern at a spatial positionfixed with respect to said diffraction wave producing means or saidsurface whereby any change in an interference pattern caused by changesin separation of said member and said surface is indicated by thedetection of interference pattern fringes moving relative to saidspacially fixed detecting position.
 4. A method according to claim 1wherein said diffraction wave producing means comprises an objectboundary.
 5. A method according to claim 1 wherein said electromagneticradiation is simultaneously directed onto a plurality of points alongsaid surface.
 6. Apparatus for determining the separtion of a surface ofa member including diffraction wave producing means comprising: meansfor directing electromagnetic radiation onto said diffraction waveproducing means of said member to form a first diffraction wave; meansfor directing electromagnetic radiation onto said surface in such amanner that incident electromagnetic radiation is reflected from saidsurface onto said diffraction wave producing means of said member, toform a second diffraction wave, said first and second diffraction wavesinteracting to form an interference pattern; and means for detecting aportion of said interference pattern for determining the separationbetween said member and said surface.
 7. Apparatus according to claim 6including detection means located at a spatial position fixed withrespect to said diffraction wave producing means or said surface wherebyany change in an interference pattern caused by changes in separation ofsaid member and said surface is indicated by the detection ofinterfereNce pattern fringes moving relative to said spacially fixeddetection means.
 8. Apparatus according to claim 6 wherein saiddiffraction wave producing means comprises an object boundary. 9.Apparatus according to claim 6 wherein said member including saiddiffraction wave producing means is elongate, an elongate gap beingformed between said member and said surface, and wherein saidelectromagnetic radiation is simultaneously directed onto a plurality ofpoints along said elongate gap.
 10. A method according to claim 1wherein said diffraction wave producing means comprises an object havingtwo boundaries.
 11. A method according to claim 1 wherein saiddiffraction wave producing means comprises a diffraction grating. 12.Apparatus according to claim 6 wherein said diffraction wave producingmeans comprises an object having two boundaries.
 13. Apparatus accordingto claim 6 wherein said diffraction wave producing means comprises adiffraction grating.
 14. A method according to claim 1 wherein saidsurface is moving.